Subtypes of dementia of the Alzheimer type (DAT)-DAT with motor abnormalities, such as extrapyramidal signs (EDAT) and myoclonus (MDAT), were studied using neurochemical measurements in the cerebrospinal fluid (CSF) . Reductions in somatostatin-like immunoreactivity (SLI) and acetylcholinesterase (AChE) activity were found in DAT, EDAT and MDAT. An inverse relation between CSF HVA and alpha-MSH-LI was found in DAT patients. Furthermore, in patients with Parkinson's disease, CSF alpha-MSH-LI was elevated. Young adult Down syndrome subjects had elevated levels of CSF choline, but AChE activity and somatostatin and neuropeptide Y concentrations did not differ from control values in either young or old Down patients. In healthy control subjects, age-related increases were found in CSF total protein, choline, the polyols erythritol and myoinositol and ACHE activity. CSF-production is reduced in healthy old subjects. The rate of formation of CSF in elderly subjects (0.2 ml/min) was 50% that of young subjects (0. 4 ml/min) . CSF Protein gradients did not differ between young and old subjects. There was no significant difference in rostrocaudal CSF gradients of somatostatin or neuropeptide Y caudal lumbar CSF fractions, whereas ACHE activity was higher in the more caudal fractions of young but not old subjects.